Re: [PATCH] [v3] vduse: fix compat handling for VDUSE_IOTLB_GET_FD/VDUSE_VQ_GET_INFO

["Arnd Bergmann" <arnd@arndb.de>]

On Wed, Jun 10, 2026, at 07:26, Michael S. Tsirkin wrote:
> On Fri, Feb 13, 2026 at 04:40:46PM +0100, Arnd Bergmann wrote:
>> +	};
>> +	__u8 ready;
>> +	__u8 padding[3];
>> +} __uapi_arch_align;
>
> what is this __uapi_arch_align supposed to be doing?
>
> It compiles by luck because gcc thinks it's a global variable.

Sorry, that should not have been part of this patch, it is
part of the series of changes I was testing when I noticed
the problem here, but that other series is unfortunately
still not ready for merging.

The background here is that I'm adding -Werror=padded to
the UAPI checks in usr/include/Makefile, in order to find
any instances of data structures with implied padding,
and then instead add explicit padding using architecture
specific macros. In vduse_vq_info, this includes a final
32 bit of padding on architectures with naturally
aligned __u64 but no padding when __u64 has a smaller
alignment.

The __uapi_arch_align macro in turn is what I add to
structures with optional padding, in order to be able
to reduce the alignment of a structure when building the
kernel with a higher default alignment than userspace
(e.g. on m68k with -malign-int).

Removing the __uapi_arch_align is the correct fix for the
moment. Let me know if you would like me to send a
replacement patch without it, a fixup patch on top, or if
you will fix it up yourself.

     Arnd